Combustion chamber

ABSTRACT

A combustion chamber  10  for a gas turbine operates in a highly diluted mode of combustion has a substantially toroidal plenum  30 . At least one combustion fluid injector is located at the periphery of the plenum. The shape of the plenum and the disposition of the combustion fuel injectors are such that a vortex flow  2  is established in the combustion chamber in use. Gasses are entrained in the vortex which provides high enough levels of the gas re-circulation to allow for the onset of highly diluted combustion.

The invention relates to a combustion chamber for a gas turbine and amethod of operating a combustion chamber for a gas turbine.

Gas turbines operate on the basis of fossil fuel combustion. Fossil fuelcombustion processes are these days governed by two major requirementswhich are in contrast with one another. On the one hand, a combustionprocess should achieve the highest possible efficiency (so as to savefuel and reduce CO₂ emissions); on the other hand, the process shouldminimise pollutant omissions (for example NOx).

One of the most common ways to improve the efficiency of a combustionprocess is to use high temperature combustion air preheating. Thisapproach causes combustion to take place at relatively high flametemperatures and eventually the energy of the high temperaturecombustion gases is transferred to the combustion air using arecuperative or regenerative heat exchanger. One drawback of highpreheated air temperatures is that the flame experiences increased peaktemperatures, with a disastrous effect upon the thermal-NOx formationpath.

Research has been carried out on the combustion of hydrocarbons usingdiluted reacting mixtures that are kept at a temperature above theself-ignition threshold via the re-circulation of flue gas. The use ofthe flue gas dilutes the reacting mixture and can be used to provide theenergy to allow for self-ignition.

Flue gas re-circulation increases the contents of inerts in a mixture.Early research into the flammability limits for combustion ofhydrocarbons and air [Zabetakis, 1965] showed that it is possible toobtain flammable mixtures for re-circulation rates of up to 50%. Morerecent research aimed at providing reliable operating conditions forpractical systems has shown that re-circulation rates of up to 30% canbe used as a NOx-reducing technique [Wilkes and Gerhold, 1980]. The fluegas recirculation rate R is defined as the ratio of the flow rate of there-circulated flue gas and the flow rate of the fresh mixture fed intothe combustion chamber:

$R = \frac{G_{IR} + G_{ER}}{F + {O\; x}}$where:

-   -   G_(IR)=Flue gas re-circulated inside the combustion chamber;    -   G_(ER)=Flue gas re-circulated outside the combustion chamber;    -   F=Fuel; and    -   Ox=fresh oxidant (usually air).

It has however recently been found that it is possible to stabilise aflame at a much higher flue gas re-circulation rate than previouslythought. This can produce a mode of combustion that produces anon-visible, non-audible flame. Such a flame is associated with eventemperature and concentration profiles, and no hot spots.

This alternate combustion mode, termed for the purposes of this documentas “highly diluted combustion”, arises as a result of the very highlevel of dilution of the reacting mixture. The high level of dilutionprevents the formation of localised temperature peaks and thus lowersNOx formation. To achieve an operating set-up that exploits theself-ignition of the flammable diluted mixture, it is necessary toprovide a mixture temperature that is above the autoignition threshold.Such a condition will result in a very low temperature differencebetween the initial and adiabatic flame temperatures, as compared toconventional non-diluted visible flames.

$T_{ad} = {{T_{i\; n} - {\frac{\Delta\; H_{R}}{c_{p}} \cdot Y_{Fuel}}} = {{- \frac{\Delta\; H_{R}}{c_{p}}} \cdot \frac{1}{R + 1} \cdot \frac{F}{F + {O\; x}}}}$${\Delta\; T} = {{T_{ad} - T_{i\; n}} \propto \frac{1}{R + 1}}$where:

-   -   T_(ad)=adiabatic temperature (K);    -   T_(in)=initial temperature of the reacting mixture (K);    -   ΔH_(R)=heat of the reaction (kJ/kg);    -   c_(p)=specific heat of reacting mixture;    -   Y_(Fuel)=molar fraction of burned fuel;    -   R=re-circulation rate;    -   F=fuel molar rate; and    -   Ox=oxidant molar rate.

The above two equations indicate that the difference between theadiabatic temperature (T_(ad)) and the initial temperature (T_(in)) ofthe mixture decreases as R increases. The re-circulation rate R acts onthe value of the initial temperature (T_(in)), as this is the result ofan energy balance between the re-circulated flue gas and the freshoxidant stream fed into the combustion chamber. However, the value of Rdoes not affect the value of the adiabatic temperature (T_(ad)), asshown from further elaboration of the above equations in conjunctionwith standard equations of adiabatic combustion:

$T_{ad} = {T_{oxi} - {\frac{\Delta\; H}{c_{p}} \cdot {\varphi(\phi)}}}$where:${\varphi(\phi)} = {{\phi \cdot \left( \frac{Y_{Fuel}}{Y_{oxi}} \right)_{stoich}} + 1}$

-   -   T_(oxi)=oxidant inlet temperature;    -   φ=equivalence ratio; and    -   Y_(oxi)=oxidant mole fraction.

The equivalence ratio parameter (φ) is frequently encountered in thestandard literature of combustion, and is simply defined as:

$\Phi = \frac{1}{\lambda}$

The relative air to fuel ratio, λ, is defined as:

$\lambda = \frac{\left( {\%\mspace{14mu}{fuel}\text{/}\%\mspace{14mu}{air}} \right)_{stoichiometric}}{\left( {\%\mspace{14mu}{fuel}\text{/}\%\mspace{14mu}{air}} \right)_{actual}}$where:

-   -   % fuel and % air are the molar percentage (or molar fraction) of        fuel and air respectively derived by:

${\%\mspace{14mu}{fuel}} = \frac{F_{Fuel}}{F_{Air} + F_{Fuel}}$${\%\mspace{14mu}{air}} = \frac{F_{Air}}{F_{Air} + F_{Fuel}}$and where:

-   -   F_(Air) and F_(Fuel) are the molar flow rates of air and fuel        respectively.

Excess air is defined as: e(%)=(λ−1)*100.

Combustion is usually characterised by the stoichiometry of the reactingmixture.

-   -   λ<1 (φ>1): fuel rich mixtures—rich stoichiometry    -   λ=φ=1: stoichiometric conditions    -   λ>1 (φ<1): fuel lean conditions—lean stoichiometry

Conventional gas turbine systems typically operate under lean premixedcombustion conditions and employ combustion chambers of the annular,can, can-annular or silos type. Such combustion systems typically relyon a swirl-stabilized flame, in which a small re-circulation zone isformed at the exit of the burners via aerodynamic means. This allowsignition and burnout in a very compact combustor zones, which results invery short residence times (of the order of a few milliseconds) andtherefore permits the use of very compact combustion chambers.

Such a system is typically operated with a very lean flame (λ≧2) ataround 20 bar, with the oxidant (usually air) preheated to 720 K bycompression, and with a flame temperature of around 1750 K. Typicalsystems have ignition delay times of the order 3 to 5 ms, with residencetimes of the order of 20–30 ms. Targeted emission levels are: UHC and CObelow 10 ppm, and single digit NOx ppm (normalised at 15% O₂). Theseexample conditions refer to a gas turbine operating in a full engineload operation mode, and it is necessary to respect the aboveconstraints.

However, such systems are associated with a number of drawbacks. Oneproblem is the generation of self-induced pressure pulsations, which canhave dramatic consequences on the mechanical integrity of the combustionsystem. This problem arises from the small re-circulation zones formedat the exit of each burner. These are not stable and can lead topressure fluctuations with the combustion chamber termed pulsations.

This tendency for the pressure to fluctuate means that it is necessaryto run such systems within constrained operating conditions. Anotherproblem still is the high risk of flashback into the burner, which is anintrinsic characteristic of lean premix systems.

As an alternative to operating with conventional lean premixswirl-stabilized flames, gas turbines could be run in a highly dilutedmode, relying on the non-stabilized auto-ignition concept. In highlydiluted combustion, a flame ignites spontaneously when enough energy hasbeen released and transferred to the reactants. Thermal energy is thentransferred to the reactants as they mix with the re-circulated fluegas. The amount of hot flue gas that needs to be mixed with reactants inorder to establish auto-ignition and burnout depends on the rating ofthe combustion process, and depends on the load for gas turbine systems.The higher the rating (and therefore the process temperatures), thelower the amount of gas dilution needed for auto-ignition and viceversa.

Implementing a highly diluted combustion mode in a gas turbine wouldallow the flame temperature to be maintained at the desired operatingvalue with a much lower difference between the adiabatic and initialtemperatures (ΔT). This would help solve the problem of suppressing hightemperature spots, and could bring benefits in terms of emissions levelsand combustion efficiency by providing a uniform temperature field.

In order to implement highly diluted combustion is a gas turbine, thesystem has to operate within the temperature and pressure rangesassociated with gas turbines, as well as their characteristictimescales. Gas turbine systems are typically run with the combustionoxidant preheated to 400–500° C. by the compression process. Therefore,no separate recuperators or regenerators are necessary.

However, heat exchangers or alternative heat sources can still be usedto further heat the combustion oxidant. As a result of their very leanstoichiometry, gas turbine systems could employ highly dilutedcombustion with partially premixed or fully premixed flames. Testsperformed at atmospheric pressure under operating conditions typical ofgas turbine systems have shown that a flue gas re-circulation ratehigher than 100% is enough to establish highly diluted combustion, dueto the very lean stoichiometry of the system (λ≧2).

In conventionally shaped gas turbine combustion chambers, highly dilutedcombustion can be established by means of high velocity jets, whosemomentum will entrain re-circulated flue gas according to the free jetmomentum law.

Chemical kinetics calculations carried out under operating conditionstypical of a gas turbine combustion systems have revealed information ona number of gas turbine parameters when operating under highly dilutedcombustion. It has been found that high flue gas re-circulation ratescause auto-ignition delay times rates that are within the rangestypically required by gas turbine systems (see FIG. 1).

The calculations also have shown that burnout times are not affected bythe dilution degree of the mixture. This is because adiabatic orquasi-adiabatic flue gas re-circulation allows for a high enough flametemperature (see FIG. 2).

A beneficial effect of flue gas re-circulation on the NOx formation pathhas been found. This effect is more pronounced at high pressure, wherecombustion carried out with a strong flue gas re-circulation rate showsbetter low-NOx potential than conventional lean premix combustion (seeFIG. 3).

It has been further found that flue gas re-circulation rate increases asthe load decreases, thus allowing hot gas entrainment at lower operatingload and allowing for auto-ignition at lower engine operating regimes(see FIG. 4). FIG. 4 shows the flue gas re-circulation rate calculatedvia the jet momentum conservation law, whereby:

$\overset{.}{\frac{M}{{\overset{.}{M}}_{0}}} = {\frac{u_{0}}{u} = \sqrt{\frac{\rho}{\rho_{0}} \cdot \frac{A}{A_{0}}}}$

For circular jets:

for:  x/d₀ < 8${K_{V}(\%)} = {\left\lbrack {{0.083*\frac{x}{d_{0}}} + {0.0128*\left( \frac{x}{d_{0}} \right)^{2}}} \right\rbrack \cdot \frac{\rho}{\rho_{0}}}$for:  x/d₀ > 8${K_{V}(\%)} = {0.32*\frac{x}{d_{0}}*\sqrt{\frac{\rho}{\rho_{0}}}}$Where:

-   -   M=total mass flow rate;    -   M₀=mass flow rate at nozzle exit;    -   u=mean axial velocity component;    -   u₀=mean axial velocity component at nozzle exit;    -   ρ=gas density;    -   ρ₀=gas density at nozzle exit;    -   A=jet cross area;    -   A₀=nozzle area;    -   x=axial distance from nozzle exit; and    -   d₀=nozzle diameter.

If the flow has a swirling component then the rate of entrainment andthe rate of velocity decay are increased. The enhanced entrainmentcapability of a swirling jet has been defined as:

$K_{V} = {\left( {{0.32 \cdot \frac{x}{d_{0}}} + {K \cdot S}} \right) \cdot \sqrt{\frac{\rho}{\rho_{0}}}}$where:

-   -   S=the swirl number;    -   K=is an empirical constant.

In a conventional gas turbine combustion process, characterized by aswirl stabilised lean premix combustion mode, the overall process timecan be defined as:τ_(tot)=τ_(tr)+τ_(ig)+τ_(BO)where:

-   -   τ_(tr)=the transport time, which in this case is the time        necessary to convey the mixture to the stabilization zone;    -   τ_(ig)=the ignition delay time;    -   τ_(BO)=the burnout time.

In a highly diluted combustion mode, the characteristic timescales canbe defined as:τ_(tot)=τ_(tr)+τ_(mix)+τ_(ig)+τ_(BO)where:

-   -   τ_(tr)=the transport time, which in this case is the time        necessary to entrain the hot flue gas;    -   τ_(mix)=the mixing time necessary to mix the oxidant diluted        with hot flue gas and the fuel.

In a conventional lean premix system the mixing time is not consideredin the equation, as the mixture is considered already perfectly premixedat the exit of the burner. However, as highly diluted combustion isestablished in conventionally shaped combustion chambers using highvelocity jets, such systems are characterized by a longer overallprocess times as the convective and mixing times are significant. Thereactants are entrained (τ_(tr)) and mixed with the hot gases (τ_(mix))as the jet develops. When enough hot gas is entrained and mixed to reachthe auto-ignition threshold, ignition will occur (τ_(ig)) and theneventually burnout (τ_(BO)).

As a result, highly diluted combustion is associated with the drawbackof a longer overall time for the process than for a conventional leanpremix system. This results in the need for longer combustion chambers,which is undesirable in gas turbine systems as a result of the augmentedmechanical stresses to the shaft.

Another drawback of using conventional gas turbine combustion chambersfor highly diluted combustion concerns the mixing process. A very gooddegree of mixing between air, fuel and the hot gas is a primaryrequirement for process performance in terms of emissions and thermalfailure control.

FIG. 5 is a schematic diagram of a conventionally shaped combustionchamber 200 being used in a highly diluted combustion mode. Highvelocity jets 250 inject compressed oxidant and fuel into the chamberand flue gas is re-circulated entirely inside the combustion chamber200. In such an arrangement, the rate of flue gas re-circulationincreases with jet velocity (or momentum). However, higher jetvelocities are also associated with higher pressure drops. Aerodynamicstudies have shown that for a typical gas turbine system the maximumre-circulation rate that can be achieved with simple high velocity jetswhile respecting the pressure drop constraints varies from 100% to 200%.

In a typical gas turbine system, the maximum pressure drop allowed forthe burner module is 3% of the total operating pressure. The use ofsingle free jets could provide re-circulation rates higher than 200%,whilst keeping the pressure drop of the burner/injector module below the3% limit. However, gas turbines typically operate with very high air tofuel ratios (i.e. very lean mixtures) and severe space constraints, andthus cannot use a burner based on single free jets. The design of highvelocity jet injectors is limited by the inherent space constraintsassociated with gas turbines and the pressure drop limit. In istherefore unavoidable that each jet will interfere with the adjacentjets and the nominal entrapment capability of each single jet will bedepleted.

Alternatively, all or part of the flue gas can be re-circulated outsidethe combustion chamber 200. This can avoid the problem of longercharacteristic times, as the re-circulated flue gas could be premixedwith the reactants before they enter the combustion chamber. However,such a configuration can result in undesirably high pressure losseswhich result in a lower process efficiency. Moreover the temperature ofthe flue gas will be lower due to heat loss during re-circulationoutside the combustion chamber. This narrows the operation flexibilityof the system and further lengthens the ignition delay and the burnouttimes.

Therefore on the basis of the above it can be difficult to provide ahigh enough flue gas re-circulation rate to allow for the onset ofhighly diluted combustion in a conventionally shaped combustion chamber.

The object of the invention is to provide a combustion chamber for a gasturbine which obviates of ameliorates the above described problemsassociated with highly diluted combustion.

According to a first aspect of the invention there is provided acombustion chamber for a gas turbine, the said combustion chambercomprising: a substantially toroidal plenum extending around a rotoraxis of the gas turbine, the said plenum comprising a substantiallytangential outlet that extends around a radially inner peripheral regionof the said plenum; wherein the said tangential outlet is adapted todirect gases towards the turbine in a direction that is substantiallyparallel to the rotor axis of the turbine; and at least one combustionfluid injector disposed in a peripheral region of the said plenum, theor each said combustion fluid injector being adapted to inject fluidinto the said plenum to establish a vortex flow in a central region ofthe said plenum that extends around the said rotor axis of the gasturbine in a circular path; wherein the or each said combustion fluidinjector is adapted to entrain sufficient gasses in the said vortex toestablish a high enough flue gas re-circulation rate to enable thegeneration of a highly diluted combustion mode in the said plenum duringcombustion.

Such a combustion chamber is able to provide high levels of flue gasre-circulation inside the combustion chamber. The levels ofre-circulated flue gas are sufficient to allow for the onset of highlydiluted combustion. In addition, such combustion chambers employ a flamethat is both compact and well distributed over the entirety of thevolume of the combustion chamber and is not associated with a flamefront. Highly diluted combustion in such chambers is achieved by meansof the vortex rather than by the high velocity jets used inconventionally shaped combustion chambers. This enables thecharacteristic timescales concerning combustion to be shorter, whichenables such combustion chambers to be more compact than conventionallyshaped combustion chambers when used for highly diluted combustion.

In one embodiment an angle formed between a direction of fluid injectionfrom the or each said combustion fluid injector and a tangentialdirection at a point where the fluid meets said vortex is from 0° to90°. In another embodiment the said angle is from 15° to 60°.

The combustion chamber may comprise at least one oxidant injector and atleast one fuel injector, the said combustion chamber being adapted tooperate with separate oxidant and fuel injection. The fuel can beinjected upstream of the oxidant injection. This can help avoidquenching and enable a large amount of control over the combustion.

The said injected fluid can comprise a premixed or partially premixedmixture of fuel and oxidant.

The or each combustion fluid injector may be characterised by a swirlcoefficient of less than 0.6. In still another embodiment the or eachcombustion fluid injector is characterised by a swirl coefficient ofless than 0.4. In another embodiment still the combustion chamberfurther comprises moving parts adapted to produce flow in the saidcombustion chamber with a swirl coefficient of greater than 0.6, andwherein the said combustion chamber is adapted to operate with a swirlcoefficient of greater than 0.6 in a back-up mode.

The combustion chamber may further comprise a plurality of out of phasepulsating jets, the said out of phase pulsating jets being adapted toinject the air and fuel into the combustion chamber.

The substantially tangential outlet may be adapted to extend around anentire radially inner peripheral region of the said plenum to define acircular outlet. The substantially tangential outlet may comprise anozzle. The combustion chamber may further comprise a guide devicelocated either upstream or downstream of the substantially tangentialoutlet, where the said guide device is adapted to direct gasses into theblades of the turbine. The guide device may comprise a vane or a blade.

According to a second aspect of the invention there is provided a methodof using a combustion chamber for a gas turbine, the method comprising:providing a substantially toroidal plenum extending around a rotor axisof the gas turbine, providing the said plenum with a substantiallyoutlet extending around an inner peripheral region of the said plenum,and using the said tangential outlet to direct gases towards the turbinein a direction that is parallel to the rotor axis of the turbine; andusing at least one combustion fluid injector disposed in a peripheralregion of the said plenum to inject fluid into the said plenum toestablish a vortex flow in a central region of the said plenum thatextends around the said rotor axis of the gas turbine in a circularpath, and using the or each said combustion fluid injector to entrainsufficient gasses in the said vortex to establish a high enough flue gasre-circulation rate to enable the generation of a highly dilutedcombustion mode in the said plenum during combustion.

In one such embodiment the method further comprises positioning the oreach said combustion fluid injector such that an angle formed between adirection of fluid injection from the or each said combustion fluidinjector and a tangential direction at a point where the fluid meetssaid vortex is from 0° to 90°. In another embodiment the method furthercomprises using an angle from 15° to 60°.

The method may further comprise using at least one oxidant injector andat least one fuel injector, and operating the said combustion chamberwith separate oxidant and fuel injection. The method can compriseinjecting the fuel upstream the oxidant injection.

The method can comprise using a premixed or partially premixed mixtureof fuel and oxidant as the injected fluid.

The method may further comprise using the or each combustion fuelinjector with a swirl coefficient of less than 0.6. In anotherembodiment the method further comprises using the or each burner with aswirl coefficient of less than 0.4. In still another embodiment themethod further comprises using moving parts to produce flow in the saidcombustion chamber with a swirl coefficient of greater than 0.6, andoperating the said combustion chamber with a swirl coefficient ofgreater than 0.6 in a back-up mode.

The method may further comprise using a plurality of out of phasepulsating jets to inject the air and fuel into the combustion chamber.

The method may further comprise using a substantially tangential outletthat extends around an entire radially inner peripheral region of thesaid plenum to define a circular outlet. The method may further compriseproviding the substantially tangential outlet in the form of a nozzle.The method may further comprise using a guide device located eitherupstream or downstream of the substantially tangential outlet to directgasses into the blades of the turbine. The method may further compriseproviding the guide device in the form of a vane or a blade.

Embodiments of the invention will now be described, by way of exampleand with reference to the accompanying drawings in which:

FIG. 1 is a graph of autoignition delay time against pressure fordifferent flue gas re-circulation rates;

FIG. 2 is a graph of minimum burnout time against pressure for differentflue gas re-circulation rates;

FIG. 3 is a graph of NOx emission against pressure that compares a leanpremixed mode with a diluted combustion mode;

FIG. 4 is a graph of re-circulation rate against load;

FIG. 5 is schematic diagram of a conventionally shaped combustionchamber adapted to be used in a highly diluted combustion mode;

FIG. 6 is a schematic view of a radial section toroidal combustionchamber according to a first embodiment of the invention;

FIG. 7 is a schematic perspective view of the toroidal combustionchamber of FIG. 6;

FIG. 8 is a schematic view of a radial section of a toroidal combustionchamber according to a second embodiment of the invention;

FIG. 9 is a schematic view of a radial section of a toroidal combustionchamber according to a third embodiment of the invention;

FIG. 10 is a schematic view of a radial section of a toroidal combustionchamber according to a fourth embodiment of the invention; and

FIG. 11 is a graph of NOx levels against temperature for both a highlydiluted flame with a re-circulation rate of 100%, and a lean premixflame.

A toroid is a solid generated by rotating a closed curve about an axis(termed the toroid axis) in its own plane. The curve does not intersector contain the axis. In an annular toroid (commonly termed a torus) theclosed curve forms a circle. For the purposes of this document, thefollowing conventions will be used. A “circumferential direction” willrefer to a direction based on a virtual circle that is coaxial with atorus. A “radial section” will refer to a section through a torus takenin the plane of the closed curve, and a “radial section circumference”is based on the perimeter of the radial section, i.e. the closed,circular curve. A “tangential direction” is defined by a tangent at apoint on the closed curve.

The combustion chamber 10 according to a first embodiment shown in FIGS.6 and 7 has an inner space 30 that has a substantially annular toroidalform that extends around the rotor axis 20 of the gas turbine, with atangential outlet that extends around the radially inner periphery ofthe inner space 30 about the rotor axis. The tangential outlet forms thecombustion chamber exit 15, and directs hot gases towards the turbine ina tangential direction that is parallel to the rotor axis 20 of the gasturbine. The outlet could be in the form of a nozzle or any suitablechannel.

The inner space 30 forms a plenum, and the combustion chamber 10comprises a fuel injector 8 and an oxidant injector 9 located at theperiphery of the toroidal inner space 30. The combustion chamber 10further comprises a turbine guide device 3 located upstream thecombustion chamber exit to direct the hot gasses from the combustionchamber 10 into the blades of the turbine. Alternatively the turbineinlet guide device 3 could be situated downstream of the combustionchamber exit 15. The turbine inlet guide device 3 could be in the formof a vane or blade or any suitable means for directing air into theblades of the turbine.

The fuel and the oxidant are injected into the toroidal inner space 30which establishes a vortex flow 2 in a central region of the toroidalinner space 30 that extends around a circular path around the rotor axisof the turbine. The angle of inclination and location of the fuelinjector and oxidant injectors are such as to entrain hot gasses tomaximize the vortex flow. The gas in the vortex flow comprises flue gasre-circulated inside the combustion chamber and around 10% residualoxygen. This is because combustion takes place at gas turbine conditions(i.e. lean stoichiometry λ=2 with residual oxygen around 10%).

The fuel is injected by the fuel injector 8 into region 11 which willtypically be at 1650 K. The fuel penetrates the hot exhaust gas withinthe vortex 2 and is mixed with the re-circulated flue gas andsimultaneously heated up to the autoignition threshold. At this point,as residual oxygen is available, the mixture ignites via an autoignitionmechanism and highly combustion is initiated with its characteristicnon-visible flame. The temperature of the process is controlled by thelevel of dilution of the mixture, and therefore the emission levels arelimited.

Oxidant is injected into the combustion chamber 10 by the oxidantinjector 9 downstream the fuel injector 8. The oxidant jet momentum willentrain hot gas while penetrating into the vortex 2 where part or all ofthe fuel is being burned or has burned.

Region 12 is located just downstream the oxidant injector, and at thispoint the hot gasses will comprise around 21% oxygen. The injection ofoxidant provides the additional oxygen necessary to ensure completeburnout of the fuel and to re-establish the required residual oxygenconcentration. Burnout will typically occur by the time the gasses reachregion 13, at which point the gasses will comprise flue gas and around10% residual oxygen. The majority of the re-circulated flue gas andresidual oxygen will then flow around the vortex back to region 11.However, some gas flowing around the outer periphery of the radialsection will converge towards the turbine inlet guide nozzle 3 and leavethe combustion chamber 10 via exit 15. The gas leaving the combustionchamber will comprise around 10% oxygen and be at around 1650 K.

Both the fuel and oxidant injectors are inclined and positioned aroundthe periphery of the toroidal combustion chamber 10 so as to optimizethe flow field inside the combustion chamber 10. In order to entrain asmuch gas as possible in the vortex, the fuel and air are injected in away to best mix with the vortex flow without generating flowdisturbances. The angle formed between the direction of the jet enteringthe combustion chamber and the tangential flow of the vortex 2 at thecorresponding location is from 0° to 90°. It has been found that theentrainment capacity of the injectors is further enhanced when the angleis from 15° to 60°.

The injectors are in the optimum location when intimate penetration andmixing of the fresh stream from the injectors with the hot gas vortex 2occurs in such a way that the gas flows along at least one complete patharound a radial section circumference of the inner space 30 beforeentering the turbine inlet guide device 3. This ensures that completecombustion takes place.

FIG. 7 shows an example of locations of the fuel and oxidant injectors.However, the fuel injector 8 could be positioned at any point around thecircumference of the toroid on line 18 and the oxidant injector could bepositioned at any point around the circumference of the toroid on line19. Furthermore embodiments of the invention can employ a plurality ofinjectors located along lines 18 and 19.

In some embodiments the injection of the fuel and/or oxidant can bestaged around a radial section circumference of the annular toroidalinner space 30 of the combustion chamber 10. This allows for gradualpenetration and mixing of the fuel into the main vortex, which can leadto greater control over the ignition and combustion. By injecting thefuel in stages, an even temperature profile can be established all overthe whole of combustion chamber 10. The oxidant can be injected instages to control the temperature and ensure that complete burnoutoccurs, and staged oxidant injection can be used to prevent any risk ofquenching.

FIG. 8 shows an embodiment of the invention that uses a number ofinjectors to inject the fuel and oxidant in stages. In this embodimentthe combustion chamber 10 comprises six fuel injectors 8 and six oxidantinjectors 9. Three of the fuel injectors 8 are located in a row around aradial section circumference of the inner toroidal space 30, with anadjacent row of three oxidant injectors 9 located further along theradial section circumference and downstream the fuel injectors 8. Theother three fuel injectors 8 and other three oxidant injectors 9 arelocated in a corresponding row located at points rotated 180° around therotor axis 20 as shown in FIG. 8.

FIG. 9 shows an embodiment that employs six fuel injectors 8 and sixoxidant injectors 9 in a different configuration. In this embodiment thefuel injectors 8 and oxidant injectors 9 are arranged in two groups,with three fuel and three oxidant injectors located in a row around aradial section circumference of the inner toroidal space 30, with eachoxidant injector 9 located adjacent and downstream of each fuel injector8. A corresponding row of adjacent fuel and oxidant injectors is locatedat points rotated 180° around the rotor axis 20. In terms of the overallcombustion chamber configuration, each row extends from a radially innerposition around a radial section circumference in a downstreamdirection.

During use, each injection of fuel is followed by the injection ofoxidant. Combustion is therefore operated in a cascade mode, whichallows for a large degree of control over combustion and burnout. Thiscascade mode establishes several small and separated autoignition andburnout zones, which has the effect of further suppressing pressurepulsations.

Alternatively, embodiments of the invention can employ otherconfigurations of fuel 8 and oxidant injectors 9. The inclination ofboth the fuel 8 and oxidant 9 injectors can be varied, and the preferredarrangement is dictated by the optimum flow field considerations.

Diffusion burners are disclosed as being a requirement in the prior artapplications of highly diluted combustion for atmospheric combustionapplications (such as for furnaces). However, in combustion chambers ofgas turbines the use of premix burners does not impede the establishmentof highly diluted combustion, provided enough flue gas is re-circulatedto allow autoignition of the mixture and sufficiently dilute the flame.Flue gas re-circulation rates of between 100% and 200% have been foundto be sufficient. All the previously described embodiments havedescribed diffusion burners with separate injection of the fuel andoxidant. This can lead to a greater amount of control over ignition andcombustion, however, in other embodiments of the invention, thecombustion chamber 10 can employ other types of burner such as premixedor partially premixed burners.

FIG. 10 is a schematic view of a radial section of a toroidal combustionchamber 10 according to a fourth embodiment of the invention. Thecombustion chamber 10 comprises three premix burners 4 located around aradial section circumference of the inner toroidal space 30. The premixburners are adapted to inject a premixed stream of fuel and oxidant intothe combustion chamber 10. Alternatively, a different number orconfiguration of burners could be used.

In all the previously described embodiments, the injection of the freshreactants at the periphery of the combustion chamber 10 and the vortex 2flow field within the chamber 10 creates a layer of colder flow in theregion of the walls of the combustion chamber 10. This can reduce thisrisk of the material comprising the walls of the combustion chamber 10overheating. In order to further reduce the risk of the walls of thecombustion chamber 10 overheating, embodiments of the invention canemploy convective cooling, in which the oxidant from the compressor isdirected through a region located adjacent the outside the toroidalinner space 30 before being injected into the combustion chamber plenum.

The injectors inject fluid with a velocity of from 80 to 100 m/s andhave a nozzle diameter of from 10 to 50 mm.

Conventional lean premix burners for gas turbine systems typicallyexploit aerodynamically induced vortex breakdown, in which astabilisation region is created and the flame anchored due to theequality between the gas convection velocity and the flame speed. Inembodiments of the present invention burners are used in which the gasvelocity always exceeds the flame speed, and a non-stabilized flame isestablished via an autoignition mechanism.

In the previously described embodiments, a non-swirled stabilized flameis used and no vortex breakdown occurs. No flame front is formed, as thecombustion chamber 10 uses highly diluted combustion relying on intimatemixing of re-circulated flue gas with the fresh reactants and onautoignition of the resulting mixture. A non-stabilized flame develops,which is characterized by not having a flame front. Instead, the flameoccupies a very diffuse volume, occupying the substantially the wholevolume of the combustion chamber 10 with a homogeneous distribution of aplurality of discrete heat sources. This gives rise to a diversecombustion mode with a very high potential for self-induced pressurepulsation suppression. It allows also for very high power density to bereached. However, in such systems the residence time distribution takesan exponential form, and some gas particles may have long residencetimes in the reactor, which can be detrimental in terms of NOx emissionproduction. This can be mitigated by the lower NOx dependence ontemperature in highly diluted conditions as shown by FIG. 11.

FIG. 11 shows a comparison of experimental results concerning a highlydiluted flame with a re-circulation rate of 100% and a standard,non-diluted flame with a re-circulation rate of zero. The experimentswere carried out with natural gas as a fuel, and with an inlettemperature of 600° C. FIG. 11 shows that the amount of NOx produced bythe highly diluted flame is less sensitive to flame temperature than thebaseline flame.

Experiments have shown that using a higher inlet temperature than 600°C. will produce an increase in the reduction of NOx associated with thehighly diluted flame. It also has the effect of enlarging thetemperature operating range at which the highly diluted combustion hasbetter NOx potential than the baseline flame.

In conventional lean premix burners for gas turbine systems, smallre-circulation zones form at the exit of each burner. These arelocalised heat sources and are not stable and can lead to pressurefluctuations with the combustion chamber termed pulsations. However, inembodiments of the present invention there is an overall re-circulationzone for all the burners. This greatly reduces the tendency for pressurefluctuation as the flame is spread over substantially the whole of thecombustion chamber 10.

However, pulsations can still occur, particularly if one of the burnersdevelops a fault and runs less efficiently than the others. In order toovercome this problem, embodiments of the invention use pulsating jetsto help control pressure pulsations. The use of out-of-phase pulsatingjets for the injection of the fuel and oxidant can be used to suppressthese pulsations if they develop. Furthermore, the use of pulsating jetscan also lead to a greater control over the process and enhanceentrainment and mixing.

The embodiments thus far described are characterised by non-swirlingflow of fuel and oxidant from their respective injectors. However,embodiments of the invention can employ a swirling component, which isknown to enhance mixing. However, too strong a swirling component willprevent the combustion chamber 10 operating in a flameless combustionmode. This is because a strong swirling component can generate aswirl-stabilised flame, which could give rise to a conventional flamefront mode with elevated flashback risk and self-induced pulsations.

However, highly diluted combustion can be used with a weak swirlingcomponent. Weak swirl is known to generate some rotational flow, withoutthe occurrence of any vortex breakdown that characterises conventionallean premixed, swirl-stabilised flames. The threshold between weak (novortex breakdown) and strong (vortex breakdown) swirl is known to beS=0.6.

Embodiments of the invention can use swirl components with S less than0.6. However, a swirl component of less that 0.4 is preferred to avoidany risk of vortex breakdown, and therefore avoid the risk of flashback.

As a back-up mode, embodiments of the present invention can additionallycomprise moving parts adapted to establish a strong (S>0.6) swirlingflow.

Combustion chambers according to embodiments of the invention solve manyof the problems associated with combustion chambers operating under leanpremix combustion. Flashback is not an issue, as flame stabilization isoperated via an autoignition mechanism.

In addition, combustion chambers according to embodiments of theinvention offer several advantages over conventionally shaped combustionchambers when used for highly diluted combustion. In particular,combustion chambers according to embodiments of the present inventionallow for higher levels of flue gas to be re-circulated inside thecombustion chamber.

In conventionally shaped chambers, flue gas re-circulation inside thechamber relies on the momentum of single free jets. However, as gasturbines typically operate with very high air to fuel ratios (i.e. verylean mixtures) and severe space constraints, a conventionally shaped gasturbine cannot use a burner based on single free jets. It is thereforeunavoidable that each jet will interfere with the adjacent jets and thenominal entrapment capability of each single jet will be depleted.

Alternatively, conventionally shaped combustion chambers can use fluegas re-circulated outside the combustion chamber. However, such aconfiguration can result in undesirably high pressure loses which resultin a lower process efficiency. Therefore on the basis of the above itcan be difficult to provide a high levels flue gas re-circulation inconventionally shaped gas turbines.

However, in embodiments of the present invention, a vortex is set upinside the combustion chamber by virtue of the an annular toroidal innerspace. Furthermore, the burners are positioned and inclined relative tothe vortex in order to maximise the entrapment of hot gasses within thisvortex. This enhanced vortex provides high enough levels of flue gasre-circulated inside the combustion chamber to allow the onset of highlydiluted combustion without the need for flue gas re-circulation outsidethe combustion chamber. However, if desired embodiments of the inventioncould re-circulate flue gas outside the combustion chamber. This fluegas re-circulated outside the combustion chamber could be premixed withthe oxidant or fuel before being injected into the combustion chamber.

In addition, the compact toroidal design allows for extremely shortrotor lengths, and does not impact on engine spatial constraints. Theflame is both compact and well distributed all over the entirety of thevolume of the combustion chamber, which allows the toroidal combustionchamber to be more compact than conventionally shaped combustionchambers when used for highly diluted combustion. Furthermore it ispossible to retrofit embodiments of the invention into an existing gasturbine systems.

The toroidal shape of combustion chambers operating in a highly dilutedmode according to the embodiments of the invention leads to veryflexible systems for dual or multi-fuel purposes as well as for engineoperation issues.

As a result of the above, embodiments of the invention are very robustand reliable, as well as being compact. This leads to increasedcombustion chamber lifetime and lower costs.

All previously described embodiments of the invention employ a toroidwith closed curve that is substantially annular, with a tangentialprojection. However, embodiments of the invention could employ toroidswith closed curves that have different curvatures. Furthermore, in alldescribed embodiments the burners have been described as located at theperiphery of the annular toroidal inner space of the combustion chamber.However, the burners could extend into the inner space.

Many further variations and modifications will suggest themselves tothose versed in the art upon making reference to the foregoingillustrative embodiments, which are given by way of example only, andwhich are not intended to limit the scope of the invention, that beingdetermined by the appended claims

1. A combustion chamber for a gas turbine, the combustion chamber comprising: a substantially toroidal plenum extending around a rotor axis of the gas turbine, the plenum comprising a substantially tangential outlet that extends around a radially inner peripheral region of the plenum, wherein the tangential outlet is adapted to direct gases towards the turbine in a direction that is substantially parallel to the rotor axis of the turbine; and at least one combustion fluid injector disposed in a peripheral region of the plenum, the at least one combustion fluid injector being adapted to inject fluid into the plenum to establish a vortex flow in a central region of the plenum that extends around the rotor axis of the gas turbine in a circular path; wherein the at least one combustion fluid injector is adapted to entrain sufficient gasses in the vortex to establish a high enough flue gas re-circulation rate to enable the generation of a highly diluted combustion mode in the plenum during combustion; and wherein the combustion chamber comprises at least two combustion fluid injectors, wherein the at least two combustion fluid injectors comprise at least one oxidant injector and at least one fuel injector, the combustion chamber being adapted to operate with separate oxidant and fuel injection.
 2. A combustion chamber according to claim 1, wherein an angle formed between a direction of fluid injection from the at least one combustion fluid injector and a tangential direction at a point where the fluid meets the vortex is from 0° to 90°.
 3. A combustion chamber according to claim 2, wherein the angle is from 150° to 60°.
 4. A combustion chamber according to claim 1, wherein the fuel is injected upstream of the oxidant injection.
 5. A combustion chamber according to claim 1, wherein the at least one combustion fluid injector has a swirl coefficient of less than 0.6.
 6. A combustion chamber according to claim 5, wherein the at least one combustion fluid injector has a swirl coefficient of less than 0.4.
 7. A combustion chamber according to claim 1, further comprising: a plurality of out of phase pulsating jets adapted to inject the air and fuel into the combustion chamber.
 8. A combustion chamber according claim 1, wherein the substantially tangential outlet is adapted to extend around an entire radially inner peripheral region of the plenum to define a circular outlet.
 9. A combustion chamber according to claim 1, wherein the substantially tangential outlet comprises a nozzle.
 10. A combustion chamber according to claim 1, further comprising: a guide device located either upstream or downstream of the substantially tangential outlet, the guide device being adapted to direct gasses into the blades of the turbine.
 11. A combustion chamber according to claim 10, wherein the guide device comprise a vane or a blade. 